MojoCompositorFrameSinkPrivate should support copy requests

cc/output/copy_output_result.h

Index: cc/output/copy_output_result.h
diff --git a/cc/output/copy_output_result.h b/cc/output/copy_output_result.h
index 23334645edcd140eb088e216fa9c8f58e56d0b91..731edb2358c860e7f0b7176993dcaeba929013cd 100644
--- a/cc/output/copy_output_result.h
+++ b/cc/output/copy_output_result.h
@@ -53,6 +53,14 @@ class CC_EXPORT CopyOutputResult {
   void TakeTexture(TextureMailbox* texture_mailbox,
                    std::unique_ptr<SingleReleaseCallback>* release_callback);
 
+  std::unique_ptr<SingleReleaseCallback> TakeReleaseCallback();
+
+  void set_release_callback(std::unique_ptr<SingleReleaseCallback> callback) {
+    release_callback_ = std::move(callback);
+  }
+
+  const gpu::Mailbox& mailbox() { return texture_mailbox_.mailbox(); }
+
  private:
   friend struct mojo::StructTraits<mojom::CopyOutputResultDataView,
                                    std::unique_ptr<CopyOutputResult>>;